Since radar level gauging was commercialized in the 1970's and 1980's, frequency modulated continuous wave (FMCW) gauging has been the dominating measuring principle for high accuracy applications. An FMCW-type filling level measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 24-27 GHz, or 9-10.5 GHz. The transmitted signal is reflected by the surface of the product in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is combined with the transmitted signal in a mixer to generate a combined signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. If a linear sweep is used, this frequency, which is also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The combined signal from the mixer is often referred to as an intermediate frequency signal or an IF-signal.
More recently, the FMCW principle has been improved, and today typically involves transmitting not a continuous frequency sweep but a signal with stepped frequency and practically constant amplitude—a stepped frequency sweep. When the transmitted and received signals are mixed, each frequency step will provide one constant piece of a piecewise constant IF-signal. The piecewise constant IF-signal is sampled and the sampled signal is transformed to the frequency plane, for example using FFT, in order to identify frequency components of the IF-signal. The frequency components may then be translated to distances, for example in the form of an echo curve or similar.
Although highly accurate, conventional FMCW systems—continuous as well as stepped—are relatively power hungry, which makes them less suitable for applications where the power (and/or energy) is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal energy source (e.g. a battery or a solar cell).
For applications with limited available energy and/or power, also the cost and size of a level measuring system are often crucial parameters. Such applications may, for example, be found in the process industry.
To allow such applications to benefit from the high performance of FMCW-type radar level gauging, it would be desirable to provide for a more compact and cost-efficient FMCW-type radar level gauge system, which is also capable of operating on the limited available power/energy with a reasonable update frequency.